Parallel probe readout for data storage

In this thesis techniques are developed to read out nanoscale probes and arrays of probes.The main targeted application area is probe-based data storage.The work also contributes to other areas, such as metrology, biological sensing, materials research and nano-electro-mechanical switches.
First, an exhaustive literature review of the accomplishments within probe storage is presented. It is found that optical readout techniques are used extensively in applications using probes; however, the very demanding application probe storage is not amongst them. Optical readout of probes offers reliability, high-speed, low noise and low complexity. It has to be extended to operation on arrays of probes for successful implementation in probe storage.
The first technique that is developed in this work is parallel frequency readout of an array of cantilever probes, demonstrated using optical beam deflection with a single laser-diode pair. Multi-frequency addressing makes the individual nanomechanical response of each cantilever distinguishable within the received signal. Addressing is accomplished by exciting the array with the sum of all cantilever resonant frequencies.This technique requires considerably less hardware compared to other parallel optical readout techniques. Readout is demonstrated in beam deflectionmode and interferencemode. Many cantilevers can be readout in parallel, limited by the oscillators’ quality factor and available bandwidth. The proposed technique facilitates parallelism in applications at the nanoscale, including probebased data storage and biological sensing.
A second technique to perform parallel optical readout of probes makes use of diffraction patterns that result if a laser spot is incident on an array of probes. The cantilevers form an optical grating and the state of deflection of each cantilever within the array determines the diffraction pattern, which is captured by a 1-dimensional array of photodiodes. Each cantilever can be regarded as a slit in a traditional multiple-slit diffraction experiment. In our situation the phase of the
reflected light is a function of the amount of deflection of the cantilever, in contrast to a slit diffraction experiment, where the slits are assumed to contain light sources
of equal phase. The developed technique is straightforward applicable when two
discrete levels are permitted in cantilever bending.
Anovel fabrication process is developed to produce probe arrayswith sharp tips
that are self-aligned on the cantilever.The focus is on achieving an array that gives rise to a highly uniform tip-medium distance. In order to accomplish this we make use of a silicon-on-insulator (SOI) wafer and define the tips by a highly uniform wet chemical etch. The fabricated micro-cantilever arrays are characterized and shown to have a high uniformity. For an array of 10 cantilevers spanning 430 µm a standard error of 11 nm is demonstrated. Furthermore, we show that it is possible
to fabricate both cantilevers and tips using a single mask.
Thee final part of this work is about scanning probe microscopy employing conductive probes, which is a powerful tool for the investigation and modification of electrical properties at the nanoscale. Application areas include semiconductormetrology,
probe-based data storage and materials research. Conductive probes can
also be used to emulate nanoscale electrical contacts. Unreliable electrical contact and tipwear have, however, severely hampered thewide-spread usage of conductive probes. In this work we introduce a forcemodulation technique for enhanced nanoscale electrical sensing using conductive probes. This technique results in lower friction, reduced tip wear and enhanced electrical contact quality. Experimental results using phase-changematerial stacks and platinum silicide conductive probes clearly demonstrate the efficacy of this technique. Furthermore, conductive-mode imaging experiments on specially prepared platinum/carbon samples are presented to demonstrate the widespread applicability of this technique.